Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a digital-to-analog converter (DAC) for converting a digital audio signal into a corresponding analog audio signal, which may be amplified and driven to a loudspeaker or other audio transducer.
One example of a DAC is a current DAC, which converts a digital audio signal into an equivalent current-mode audio signal. A current DAC may be single-ended, generating a single-ended current audio signal, or may be differential, generating a differential audio signal. In its simplest form, a differential current DAC may comprise a collection of switched current sources wherein each current source is switched between one of the differential outputs or a common-mode ground. The current sources may be fractionally weighted relative to each other (e.g., each current source may be weighted by a power of two relative to other current sources), and the output current on each of the differential outputs may be the sum of current flowing into and out of the output. In many implementations, each current source may be embodied as a scaled mirror from a reference leg current source of the current DAC. As a result, noise in such reference leg may be a critical parameter of the current DAC performance, as the mirroring ratio of the reference leg to each of the switched current sources may mirror and multiply such noise. To avoid the dominance of such noise, very low mirroring ratios may be established between the switched current source and the reference leg current source. However, such low mirroring ratios may lead to considerable consumption of power of the reference leg current, such that the reference leg becomes the dominant component of power consumption of the current DAC. Accordingly, systems and methods are desired to overcome this limitation in order to minimize both noise and power consumption in current DACs.